1. Field of the Invention
The present invention relates to an improved method for recovery of lead, in its elementary form, that is to say in the form of metal, from lead compounds present in an exhausted lead-acid storage battery.
2. Description of the Prior Art
The improved method according to the present invention is based on a series of redox reactions between the lead oxides and sulphate with metallic iron. Thanks to these reactions the lead components are reduced to an oxidation state of zero, whereas the iron is oxidised to oxidation state 2 and 3. It is known that storage batteries, or secondary cells, have the ability to store electric energy, transforming it into chemical energy during charging and returning it, by means of inverse transformation, during discharge.
During discharge the storage battery acts as a battery in which a redox reaction takes place, and during charging it acts as an electrolyte cell in which the opposite redox reaction takes place.
The reactions that take place on the electrodes during discharge are the following:
on the positive electrode (cathode) PbO.sub.2 +3H.sub.3 O.sup.+ +HSO.sup.-.sub.4 +2e.sup.- .fwdarw.PbSO.sub.4 +5H.sub.2 O PA1 on the negative electrode (anode) Pb+HSO.sup.-.sub.4 +H.sub.2 O.fwdarw.PbSO.sub.4 +H.sub.3 O.sup.+ +2e.sup.- PA1 global reaction PbO.sub.2 +Pb+2H.sub.2 SO.sub.4 .fwdarw.2PbSO.sub.4 +H.sub.2 O E.sub.o =2.2V When charging, the reactions are inverted, to give 2PbSO.sub.4 +2H.sub.2 O.fwdarw.PbO.sub.2 +Pb+2H.sub.2 SO.sub.4 PA1 In a new storage battery the active components are made up of approximately 50% of metal lead and 50% of lead dioxide PbO.sub.2. PA1 uniting the solids separated in steps a) and b), essentially comprising all the lead (IV) oxide, possibly a part of lead (II) oxide, metal lead in a pulverous state and the excess of metal iron; PA1 mixing the solids united in this way with solid, liquid and/or gaseous reducing agents; PA1 treating the mixture obtained in this way, at a temperature comprised between 400 and 1300.degree. C., optionally under stirring, to form molten metal lead in the presence of a small amount of slag and without the emission of sulphurous and sulphuric gases. PA1 in the hydrometallurgy stage of the method according to the present invention all the lead sulphate is treated, amounting to over 50% by weight of the pastel, with a minimum expenditure of energy as the reaction is carried out at a low temperature, for example 80.degree. C.; PA1 in the heat stage of the method the lead produced in the hydrometallurgy section is melted down, an operation requiring a minimum energy expenditure as this element has a low specific heat (0.038 cal/g .degree.C.) 1!, and the lead (II)oxide is reduced, as the lead (IV) oxide, when heated, without the addition of any reducing agent, to a temperature of over 510-640.degree. C. 2!, spontaneously releases oxygen, reducing itself to lead (II) oxide. This oxide can be reduced both quickly and easily to metal, using coal, CO, hydrogen and hydrocarbons, at a temperature of between 300 and 600.degree. C. depending on the type of reducing agent being employed 3-4!. This gives further noticeable advantages from an energy saving point of view, both due to the low reaction temperature and due to the low speed of the reaction itself. During treatment of the pastel, as the latter is made up of non-pure products containing a high level of impurities, it is necessary to work at higher temperatures to encourage formation of waste products that contain these impurities. PA1 in the heat stage there is no treatment of the lead sulphate, a product that in order to be converted into metal lead requires the presence of a large excess of iron, necessary to transfer the sulphate anion, and long contact periods with high reaction temperatures. PA1 elimination of the emissions of sulphurous and/or sulphuric gas connected with the reaction of lead sulphate at high temperature; PA1 lower emission of carbon dioxide, as the method according to the present invention requires lower amounts of energy; PA1 drastic reduction of the production of waste, as the consumption on both sodium carbonate and glass is reduced to a negligible amount.
The storage battery is not susceptible to further re-charging, as the initial levels of the two components mentioned above are for the most part converted into lead sulphate in an electro-chemically irreversible manner.
In these conditions both the types of oxidised lead are present both on the electrodes and in the acid sludge resulting from the destructive processes that occur in the electrolytic material during operation of the storage battery.
Exhausted lead-acid storage batteries, which until recently were considered a dangerous source of pollution due to the presence of lead compounds and sulphuric acid, are now an important source of lead metal, especially when considering the progressive exhaustion of the mineral resources relating to this metal.
Increasingly restrictive legislation for the control of pollution have enforced complete recovery of the lead compounds found in these batteries. In Italy alone, 150 thousand tons these compounds are recovered every year. This figure corresponds to approximately 94% of the total number of exhausted storage batteries.
It is known that the current state of evolution of technology relating to the recovery of lead from exhausted storage batteries, in Italy for example, exclusively involves the use of pyrometallurgic processes. However, the use of these processes is unsatisfactory, both from an economic and from an ecological point of view. They are in fact processes that operate at high temperature directly on the lead compounds in the storage batteries. This technological need to operate at temperatures in the order of 1200-1400.degree. C inevitably results in the formation of sulphur and sulphuric dioxide, the emission of lead vapours that are both difficult and expensive to reduce, and the formation of lead chloride by interaction with chlorinated plastic materials that are present in the form of waste coating elements.
In this brief summary of the state of the art, it is also necessary to mention that the methods--for recovery of lead from exhausted lead-acid storage batteries--based on electrolytic processes have been abandoned because of their excessively high costs, in spite of the fact that they resulted in very low emission levels.
There is therefore a need in this specific field for a new method for recovery of lead from exhausted lead-acid storage batteries, that is both simple and versatile from a technological point of view, low in cost and non-polluting.
This need is completely satisfied by the new method according to the present invention, which also involves additional advantages which will be made clear in the following.